A coating is a covering that is applied to the surface of an object, usually referred to as the substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquids, gases or solids e.g. Powder coatings.

Paints and lacquers are coatings that mostly have dual uses of protecting the substrate and being decorative, although some artists paints are only for decoration, and the paint on large industrial pipes is for preventing corrosion and identification e.g. blue for process water, red for fire-fighting control etc. Functional coatings may be applied to change the surface properties of the substrate, such as adhesion, wettability, corrosion resistance, or wear resistance. In other cases, e.g. semiconductor device fabrication (where the substrate is a wafer), the coating adds a completely new property, such as a magnetic response or electrical conductivity, and forms an essential part of the finished product.

A major consideration for most coating processes is that the coating is to be applied at a controlled thickness, and a number of different processes are in use to achieve this control, ranging from a simple brush for painting a wall, to some very expensive machinery applying coatings in the electronics industry. A further consideration for ‘non-all-over’ coatings is that control is needed as to where the coating is to be applied. A number of these non-all-over coating processes are printing processes. Many industrial coating processes involve the application of a thin film of functional material to a substrate, such as paper, fabric, film, foil, or sheet stock. If the substrate starts and ends the process wound up in a roll, the process may be termed “roll-to-roll” or “web-based” coating.

Coatings are not just designed to be aesthetically pleasing and for decorative purposes, but also have other functions. Sometimes a coating can be both decorative and have a specific function. An example would be the coating of a pipe carrying water for a fire suppression system that is coated with a red (for identification) anticorrosion paint to reduce degradation. In fact, most surface coatings or paints are to some extent protecting the substrate e.g. general maintenance coatings/paints for metals and concrete. The decorative aspect of coatings is not just to impart a specific color, but also to create a particular reflective property such as high gloss, satin or flat/matt appearance. Some coatings though, are specifically designed to be very chemically resistant.

A major use of coatings is to protect metal, and these are generally known as anticorrosion coatings. This use includes preserving machinery, equipment and structures. Automobiles have improved in design over the years. Most are still made of metal for crashworthiness. The external coating and the underbody are coated.

Coatings are also used to seal the surface of concrete. This would include Seamless polymer/resin flooring, bund wall/containment lining, Waterproofing and damp proofing of concrete walls, and concrete bridge decks.

Roof coatings have been developed and improved over the years. They are designed primarily for waterproofing and also sun reflection to help keep a building cool. They tend to be elastomeric to allow for movement of the roof without cracking the coating membrane.

The coating, sealing and waterproofing of wood has been going on since biblical times, with God commanding Noah to build an ark and then coat it. Wood was and is a key material of construction since ancient times so its preservation by coating has received much attention. Efforts to improve the performance of wood coatings continues.

Coatings are used to alter tribological properties and wear characteristics. Other functions of coatings include :

  • UV coatings
  • Anti-reflective coatings for example on spectacles.
  • Non-stick PTFE coated cooking pots/pans.
  • Optical coatings are available that alter optical properties of a material or object.
  • Anti-Friction, Wear and Scuffing Resistance Coatings for Rolling-element bearings
  • Coatings that alter or have magnetic, electrical or electronic properties.
  • Antimicrobial coatings.
  • Anti-fouling coatings
  • Flame retardant coatings.

Coating analysis and characterization

Numerous destructive and non-destructive evaluation (NDE) methods exist for characterizing coatings. The most common destructive method is microscopy of a mounted cross-section of the coating and its substrate. The most common non-destructive techniques include ultrasonic thickness measurement, X-ray fluorescence (XRF), X-Ray diffraction (XRD) and micro hardness indentation. X-ray photoelectron spectroscopy (XPS) is also a classical characterization method to investigate the chemical composition of the nanometer thick surface layer of a material. Scanning electron microscopy coupled with energy dispersive X-ray spectrometry (SEM-EDX, or SEM-EDS) allows to visualize the surface texture and to probe its elementary chemical composition. Other characterization methods include transmission electron microscopy (TEM), atomic force microscopy (AFM), scanning tunneling microscope (STM), and Rutherford backscattering spectrometry (RBS). Various methods of Chromatography are also used, as well as thermogravimetric analysis.

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